"We know the keepers heard a yell, and they ran over and saw the baby had been injured," zoo spokeswoman Sharon Dewar said Monday night.

The veterinary staff separated Nayembi, who was born Nov. 16, and her mother, Rollie, from the rest of the gorillas and performed surgery on the baby soon after at the zoo's on-site hospital, Bell said.

The building was not opened to visitors when the injury occurred.

"I think the key thing is that our staff is prepared for every type of situation," Dewar said.

Nayembi seems to be in stable condition, but Bell said the health of younger animals is hard to predict.

"Her recovery isn’t a certainty — we find few certainties with such fragile infants, nearly a quarter of whom don’t make it through their first year of life," Bell said. "But she’s playing throughout the day and getting plenty of sleep, just like an infant should. These are good signs."

Bell said there were no prior indications of aggression in the gorilla troop, but following the incident Zoo staff has increased monitoring of the group.

Following last week's injury, Nayembi's mother, Rollie, returned to the troop. Nayembi will remain separated to receive full-time care in the coming months, Bell said.

Zoo officials said there are no video cameras inside the exhibit that could have documented that incident.

In November 2011, a 9-day-old gorilla that had yet to be named died at the zoo from a head trauma. Bana, the gorilla's mother, was found carrying her dead baby by trainers at the zoo.

Officials said they were unsure of how the accident happened, but it occurred overnight.

"It’s not uncommon unfortunately for baby gorillas to not make it past the first couple of weeks," said Tiffany Ruddle, a spokesperson for the zoo, when Nayembi was born in November.

Nayembi was the 52nd baby gorilla born at the zoo, and the last time Lincoln Park Zoo had two infants in the same troop was 2004, when Susie and Azizi were born a year apart.